This application is related to processes suitable for producing nanostructures in the form of quantum dots (“QD-s”) in silicon (Si). This disclosure is more particularly related to a new process for fabricating nanostructures in the form of QD-s in epitaxially grown silicon.
Quantum dots form when very thin semiconductor films buckle due to the stress of having lattice structures slightly different in size from those of the material upon which the films are grown.
The small size of QD-s results in new quantum phenomena that yield desirable effects. Material properties change dramatically because quantum effects arise from the confinement of electrons and holes in the material. The small size changes other material properties such as the electrical and nonlinear optical properties of a material, making them very different from those of the material's bulk form. Hence, these very small, semiconducting quantum dots provide gateways to an large array of possible applications and new technologies.
Light emission in bulk silicon presents a challenge because Si is an indirect bandgap material, therefore radiative transition requires phonon assistance, and therefore the transition has low probability so that non-radiative processes dominate. A phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid.
Having quantum dots allows overcoming these difficulties by broadening the electron and hole wave functions in momentum space, thereby increasing their overlap in the Brillouin zone, thus allowing direct radiative recombination, and increasing radiative process probability. The Brillouin zone is a property of a crystal whose geometrical shape can be considered to contain the valence electrons of the crystal. Its planes define the location of the band gap in momentum space (k-space).
Difficulties with QD-s previously realized include the fact that QD-s were embedded in an SiO2 matrix, which provides poor conductivity for electrical pumping. Reducing QD separation to improve conductivity (e.g., by tunneling) compromises the dot isolation required for electron/hole quantum confinement, and therefore compromises optical performance. Similar problems with charge transport are encountered in the case of micro-porous Si. In addition, a very thin layer of SiO2 with embedded QD-s is required to maintain low threshold voltage for luminescence, however, having a thin layer means few dots and thus low brightness. Careful passivation of QD surfaces is required to improve stability, but this impairs electron transport (for pumping). Lowering the resistance of the matrix material to improve conductivity appears to lead to the current bypassing QD-s, resulting in compromised pumping efficiency. Further, variation in QD size contributes to line broadening, and further processing is required to place QD-s in an optical cavity as would be required for producing a laser.
What is needed is a process for producing quantum dots which overcomes the above-mentioned problems, and which may be implemented using conventional semiconductor processing techniques.